The invention relates to an anti-skid brake control system.
As it can be noted from the publication AT Z 71 (1969/issue 6, pages 181-189, esp. p. 184), the use of the difference D between vehicle deceleration and wheel deceleration as a control value in an anti-skid brake control system has already been realized. However, this type of control has not proven itself.
During braking under anti-skid conditions it is of significant importance to identify at what point in time the wheel slippage exceeds the maximum of the .mu. slippage curve. If the identification is made too late it is possible that the pressure in the wheel brake cylinder is not reduced fast enough so as to prevent a locking of the wheel. The earlier this identification is made the earlier the pressure can be reduced. An early identification permits maintaining a low difference between actual slippage and maximum permissible slippage than does an identification at a later point. Numerous external conditions, however, such as uneven roads and similar road conditions complicate the exact identification of when the wheel speed falls below the instability limit (i.e. the wheel speed which belongs to the maximum of the tire slippage curve). The wheel speed measure is therefore usually filtered first and after being filtered used for determining the instability.
A further aid to timely identify instability would be means to predict instability. This can be achieved by estimating the stability reserve.
If the wheel slippage is smaller than the one belonging to the maximum of the wheel slippage curve the wheel is within the range of stability of the slippage curve. Provided the slippage is within the stability range of the slippage curve, the difference between the actual tire slippage and the tire slippage which belongs to the maximum of the .mu. slippage curve is subsequently referred to as the stability reserve. The greater the difference is, the greater is the stability reserve. The greater the stability reserve is, the greater is the difference between the present wheel brake pressure and the wheel brake pressure which belongs to the maximum of the slippage curve. On the other hand, the smaller the stability reserve is, the closer is the wheel brake pressure to the pressure which corresponds to the maximum of the wheel slippage curve.
As noted above in addition to the wheel brake pressure the wheel is also affected by factors such as uneven roads and other interfering conditions. If the wheel slippage is within the range of stability of the slippage curve, far away from the instability limit (i.e. there is a great stability reserve) these interferences lead to smaller wheel slippage changes than when the stability reserve is small. Hence, the interferences of the wheel provide information regarding the stability reserve. The faster the wheel slippage change caused by an interference approaches normal values again the greater is the stability reserve.